This application claims priority under 35 U.S.C. Sec. 119 to No.2000-227924 filed in Japan on Jul. 27, 2000, the entire content of which is herein incorporated by reference.
1. Field of the Invention
The present invention relates to a front-rear braking force distribution control system for controlling the braking force applied to rear wheels to be in a certain relationship with the braking force applied to front wheels in accordance with a state of braking operation of a vehicle.
2. Description of the Related Arts
In general, when a moving vehicle is braked, axle loads applied to the front and rear portions of the vehicle respectively will be different from each other due to the moving load caused by the braking operation. Therefore, the braking force applied to front wheels and the braking force applied to rear wheels for locking all of the wheels simultaneously are not in direct proportion to each other, but in such a relationship as called an ideal braking force distribution, which may be varied depending upon the presence or absence of the load on the vehicle.
In this respect, Japanese patent Laid-open Publication No.58-199259 discloses a hydraulic braking pressure control apparatus, which includes a pressure control valve disposed in a brake pipe for rear wheels for restraining the hydraulic braking pressure supplied to the rear wheels, and a rotational speed detection device for detecting a state that the rotational speed of the rear wheel has become less than the rotational speed of the front wheel, to actuate the pressure control valve. According to that apparatus, therefore, when the rotational speed of the rear wheel has become less than the rotational speed of the front wheel, the hydraulic braking pressure applied to the rear wheels is limited, to prevent the rear wheels from being locked.
Furthermore, Japanese patent Laid-open Publication No.9-011878 proposes a method of braking force distribution control for a vehicle, wherein unnecessary braking force distribution control caused by a road surface variation or the like is avoided, by providing a necessary condition that a vehicle deceleration exceeds a reference value thereof, when the braking force distribution control is performed. In that Publication, it is described that the unnecessary braking force distribution control can be avoided, by performing the braking force distribution control, when the vehicle deceleration exceeds a first reference value of the vehicle deceleration, in such a condition that a difference between the front and rear wheel speeds is shifted from a target wheel speed difference. It is also described that, when the vehicle deceleration exceeds a second reference value of the vehicle deceleration which is greater than the first reference value, the braking force distribution control may be performed irrespective of the difference between the front and rear wheel speeds, whereby the braking force distribution control can be begun immediately, even in the case where the variation of the difference between the front and rear wheel speeds is delayed due to influence of a pitching motion of the vehicle body to be varied gradually in a rapid braking operation.
According to the front-rear braking force distribution control apparatus as described in the Publication No.58-199259, when the brake pedal is depressed gradually, the braking force applied to the front and rear wheels is distributed properly in accordance with a desired ratio. However, when the brake pedal is depressed more rapidly than a load shift in a normal braking operation, i.e., when the rapid operation of the brake pedal is performed, then a control for distributing the braking force applied to the front and rear wheels will be delayed. This is because when the brake pedal is depressed rapidly, the front wheel cylinder pressure will reach a pressure for enabling the front wheel to be locked before a desired load shift will occur, to result in a large drop of the front wheel speed, so that the condition for beginning the braking force distribution control will not be satisfied, although it is generally constituted that the braking force distribution control will begin when the rear wheel speed becomes less than the front wheel speed by more than a predetermined value. Thereafter, the load will be shifted to the front wheel to reduce the rear wheel speed, and when the rear wheel speed becomes less than the front wheel speed by more than the predetermined value, the braking force distribution control will finally begin. In the Publication No.58-199259, however, nothing is disclosed about the rapid operation of the brake pedal as described above.
And, the apparatus as disclosed in the Japanese patent Laid-open Publication No.9-011878, it is disclosed that when the vehicle deceleration exceeds the reference value thereof, the braking force distribution control may be performed. In view of the fact that the vehicle deceleration is increased by depressing the brake pedal, the rapid operation of the brake pedal can be detected by the rapid increase of the vehicle deceleration. According to the apparatus as disclosed in the Publication No.9-011878, therefore, the braking force distribution control can be begun earlier than the apparatus as disclosed in the Publication No.58-199259. However, it is still insufficient, especially for a so-called multi utility vehicle having the gravity center thereof placed at a relatively high position, which is now quite popular, the load shift is relatively large, so that the pitching motion may be caused when braking. With respect to the vehicle having the high gravity center, therefore, it is preferable to avoid using the prior apparatus or method as described in those publications.
Accordingly, it is an object of the present invention to provide a front-rear braking force distribution control system for use in various vehicles including a vehicle having the gravity center placed at a relatively high position, to perform the front-rear braking force distribution control properly.
In accomplish the above and other objects, a front-rear braking force distribution control system includes wheel brake cylinders operatively associated with front and rear wheels of a vehicle for applying the braking force thereto, respectively, a pressure generator for pressurizing brake fluid in response to operation of a brake pedal to generate a hydraulic braking pressure and supply the hydraulic braking pressure to each of the wheel brake cylinders, and a pressure control device disposed between the pressure generator and the wheel brake cylinders for controlling the hydraulic braking pressure in each of the wheel brake cylinders. A pressure detection device is provided for detecting the hydraulic braking pressure generated by the pressure generator. A comparison device is provide for comparing the hydraulic braking pressure detected by the pressure detection device with a predetermined threshold pressure. And, a braking force control device is provided for controlling the pressure control device to adjust the braking force applied to the rear wheels in accordance with a predetermined relationship with the braking force applied to the front wheels to perform the front-rear braking force distribution control. The braking force control device is adapted to provide conditions for beginning the front-rear braking force distribution control, including at least a condition for beginning the front-rear braking force distribution control performed by controlling the pressure control device, when the comparison device determines that the hydraulic braking pressure detected by the pressure detection device exceeds the predetermined threshold pressure.
Preferably, the system may further include an increasing rate determination device for determining a pressure increasing rate of the hydraulic braking pressure detected by the pressure detection device, and a threshold pressure setting device for setting the predetermined threshold pressure in accordance with the pressure increasing rate determined by the increasing rate determination means.
The system may further include a vehicle deceleration detection device for detecting a vehicle deceleration of the vehicle. And, it may be so arranged that the comparison device compares the hydraulic braking pressure detected by the pressure detection device with the predetermined threshold pressure, and compares the vehicle deceleration detected by the vehicle deceleration detection device with a predetermined threshold deceleration, and that the braking force control device provides the condition for beginning the front-rear braking force distribution control performed by controlling the pressure control device, when the comparison device determines that the hydraulic braking pressure detected by the pressure detection device exceeds the predetermined threshold pressure, and that the vehicle deceleration detected by the vehicle deceleration detection device exceeds the predetermined threshold deceleration.
Furthermore, the system may further includes wheel speed sensors for detecting wheel speeds of the front and rear wheels, respectively, and a vehicle deceleration calculation device for calculating a vehicle deceleration of the vehicle on the basis of the wheel speeds detected by the wheel speed sensors. The braking force control device is preferably adapted to provide the condition for beginning the front-rear braking force distribution control performed by controlling the pressure control device, when the braking force control device determines that at least one of the rear wheels is in a slip condition on the basis of the wheel speeds detected by the wheel speed sensors, and determines that the vehicle deceleration calculated by the vehicle deceleration calculation device exceeds a predetermined deceleration, in the case where the comparison device determines that the hydraulic braking pressure detected by the pressure detection device is equal to or less than the predetermined threshold pressure.